Switches are well known in the art of circuit design. The most common types of switches used in circuit design are the NMOS switch, the PMOS switch, and the CMOS switch. The on-resistance (Ron) of switches is known to vary. This variation in on-resistance in turn leads to distortion on the signal as it passes through the switch. To minimise Ron variations, the gates of NMOS switches are usually biased to be at the highest available voltage, typically, VDD. FIG. 2 shows a typical Ron plot for a prior art NMOS switch in the on state, with Ron plotted against the signal voltage of the NMOS switch. As the signal voltage approaches VDD the Ron becomes so high that the signal on the output of the switch no longer follows the input and instead starts to be clamped.
In contrast, in PMOS switches the gate is usually biased to be at the lowest available voltage, typically ground, to minimise Ron variations. FIG. 6 shows a typical Ron plot for a prior art PMOS switch in the on state. Ron is plotted against the signal voltage of the PMOS switch. As the signal voltage approaches ground, Ron becomes so high that the signal on the output of the switch no longer follows the input and instead starts to be clamped, which results in distortion of the signal as it passes through the switch.
A CMOS switch comprises an NMOS transistor coupled in parallel to a PMOS transistor, thus advantageously combining the characteristics of NMOS and PMOS devices. Although the Ron characteristic is improved over the Ron of an NMOS switch or the PMOS, Ron still displays significant variation, as illustrated in FIG. 9. This variation in resistance results in distortion of the signal passing through the switch.
Other switching arrangements are also available. For example, switches that include a field effect transistor (FET). A FET switch functions by receiving a control (or gate) bias which is sufficient to cause the FET channel to conduct in an “on” state and not to conduct in an “off” state. Such switches can be constructed, for example, using metal oxide semiconductor field effect transistors (MOSFETs), junction field effect transistors (JFETs) and metal-semiconductor field effect transistors (MESFETs). A FET switch can be used in discrete and integrated circuits as voltage controlled switches either connecting or disconnecting the source from the drain of the FET device.